This invention relates to precipitation strengthenable, nickel-cobalt-iron base alloys and articles made therefrom that contain chromium, and in particular, to such an alloy and article in which the elements are balanced to provide a unique combination of controlled thermal expansion, elevated temperature oxidation resistance, strength, and ductility.
Precipitation strengthenable, controlled thermal expansion alloys have been used in apparatus in which close tolerances must be maintained at high operating temperatures, such as in jet aircraft engines and gas turbines, because they provide a combination of high strength and low thermal expansion properties that such uses demand. The high temperatures to which the known controlled thermal expansion alloys are exposed in use, e.g., up to 1000 F., are expected to become still higher, e.g., 1200 F. and above. The oxidation resistance and the stability of the gamma prime phase of the known controlled thermal expansion alloys become inadequate at such higher operating temperatures, and can result in shorter useful life of parts made from such alloys.
Protective coatings have been used to prevent catastrophic oxidation of the known controlled thermal expansion alloys at temperatures above 1000 F. A disadvantage of using protective coatings is that known coatings must be applied at high temperatures, e.g., 1550-1750 F., and exposure of the alloys to such temperatures limits the attainment of desired mechanical properties when the alloys are subsequently age hardened. The process of applying protective coatings often results in an undesirable amount of scrap material because of such defects as warpage or distortion of parts during the coating process.
Experience with the known precipitation strengthenable, controlled thermal expansion alloys has shown that they undergo a significant loss in strength when exposed for extended periods of time to the high temperatures and loads that are typical of commercial jet engines or gas turbines. This loss in strength is primarily attributable to transformation of the gamma prime strengthening phase to eta phase and/or epsilon phase, which are significantly less effective for providing the high strength desired in such alloys.
Accordingly, a need has arisen for a high-temperature alloy that provides low thermal expansion, high strength, good ductility, especially, good stress rupture ductility, and good oxidation resistance at temperatures up to about 1200.degree. F. without requiring the application of a protective coating. Additionally, it is also desirable to provide good thermal stability in such an alloy. Here and throughout this application, the term "thermal stability" refers to the ability of the gamma prime strengthening phase to resist transformation at elevated temperatures and loads.
U.S. Pat. No. 4,066,447 ('447) relates to a nickel-iron base alloy that contains a small amount of chromium for the stated purpose of overcoming "certain difficulties of obtaining satisfactory notch strength, particularly 1200 F. notch-rupture strength, in high-strength low-expansion nickel-iron alloy product strengthened with gamma-prime precipitates . . . ". The broad composition of the alloy set forth in the '447 patent is as follows, in weight percent:
C 0-0.20 PA1 Mn 0-2 PA1 Si 0-1 PA1 P 0-0.015 PA1 S 0-0.015 PA1 Cr 1.7-8.3 PA1 Ni 30-57 PA1 Mo 0-1 PA1 Co 0-31 PA1 Ti 1-2 PA1 Al 0-1.5 PA1 Nb 1.5-5 PA1 Zr 0.10 PA1 B 0-0.03 PA1 Cu 0-1 PA1 W 0-1 PA1 C 0.03 max. PA1 Mn 0-2 PA1 Si 0-0.5 PA1 P 0.015 max PA1 S 0.015 max. PA1 Cr 0-6.2 PA1 Ni 34-55.3 PA1 Mo 0-1 PA1 Co 0-25.2 PA1 Ti 1-2 PA1 Al 0.20 max. PA1 Nb+1/2Ta 1.5-5.5 PA1 Zr 0-0.1 PA1 B 0-0.03 PA1 Cu 0-1 PA1 W 0-1 PA1 Mn+Cr 6.2 max.
and the balance is iron in an amount of at least 34 weight percent. The composition of the alloy is controlled to satisfy four relationships, A., B., C., and D. set forth in column 2, lines 52-61 of the patent. U.S. Pat. No. 4,200,459 ('459) relates to a nickel-iron base alloy which can contain up to 6.2% chromium. As characterized in the patent the alloy provides "controlled thermal expansion coefficient and inflection temperature and...high strength in [the] age-hardened condition and has [a] composition specially restricted to overcome detrimental sensitivity to stress-concentrating geometries and aid resistance to long-enduring stress in heated oxidizing atmospheres." The broad composition of the alloy set forth in the '459 patent is as follows, in weight percent:
and the balance is iron in the range of about 20-55%. The composition of the alloy is controlled to satisfy three relationships, A, B, and C set forth in column 2, lines 32-37 of the patent.
Although the '447 and '459 patents describe nickel-iron base alloys which contain chromium and may contain cobalt, and refer to low thermal expansion and high strength properties, they leave much to be desired in meeting modern day requirements for a good combination of low thermal expansion, high strength, good ductility, thermal stability, and good oxidation resistance at temperatures up to 1200 F. or above.